High pressure sodium vapor lamp



Apnl 26, 1966 K. SCHMIDT 3,248,590

HIGH PRESSURE SODIUM VAPOR LAMP Filed March 1, 1963 5 Sheets-Sheet l CONTA INS SOD/UM v i 1 J 1 l0 \ALUMINA CERAMIC apnea 444v MAY 111.50 l Comm/1v l/Vf/PT 6A5 e 7. 145A?) Xe MERCMQY r/mzumw 5 g Z/(/0am J5) I m i g 1 g 20(jamp5) g k u 5 1 m x 1 g y S i 1 E 3 0 I00 200 300 400 500 600 700 800 900 I000 C Ter nperazure of Saturazed Vapor Sod/urn Vapor Pressure .j. INVENTOR. Kurc Schmidt M2W v His Attorney April 26, 1966 K. SCHMIDT 3,248,590

HIGH PRESSURE SODIUM VAPOR LAMP Filed March 1, 1963 5 Sheets-Sheet 2 Na. Przssu/e a 0.3mm H i. L A k A 4 Na Wassune. 4010? H9 U U L J W Na Pressure: C 20mm Hy A JUU LAW A/a Pressure: 2000107161 MW/ L L M, A 7000 6000 5000 4000 WAVE LENGTH //v fl/vesmoM UNITS INVENTOR. E5 Rur c Schmidt His Attorneg 5 Sheets-Sheet 3 April 26, 1966 K. SCHMIDT HIGH PRESSURE SODIUM VAPOR LAMP Filed March 1, 1963 I i 5% 4.

200 400 600 600 I moo Sop/0M M01 02 P2555025 f/v mm Hg r-50m GHT F/LL 643 200177171 0f XENON FQB.

5'0D/UM Va oe P2555025 mm Hy Sop/UM 1 0/ 012 PRESSURE mm Hg lfivervlr'ofi Hur- 17 S c hm id lr b H 5.7%?

Hi 5 A L- CL'OTTWSH United States Patent 3,248,590 HIGH PRESSURE SODIUM VAPOR LAMP Kurt Schmidt, Cleveland Heights, Ohio, assignor to Gencral Electric Company, a corporation of New York Filed Mar. 11, 1963, Ser. No. 263,676 15 Claims. (Cl. 313-184) This invention relates to a metal vapor lamp ultilizing sodium as the principal component of the discharge supporting medium and producing a radiant emission resulting in a White or nearly White color. This application is a continuation-in-part of my copending application Serial No. 88,183, filed February 9, 1961, entitled Sodium Vapor Lamp, now abandoned.

The conventional sodium vapor lamp which is used extensively for outdoor applications in some European countries, and to a lesser extent for highway lighting in this country, is of the low pressure type. In these lamps, the sodium vapor pressure is of the order of a few microns, corresponding to a temperature of the sodium metal within the lamp in the range of 200 to 300 C., and the current density is low, generally less than 1 ampere per square centimeter of discharge cross section. The optimum vapor pressure for maximum output of resonance radiation is about 0.9 micron of mercury, corresponding to a sodium metal temperature of about 220 C. The emission of the low pressure sodium vapor lamp is composed of sharp lines rather than continuous radiation. Most of the radiated energy is concentrated in the characteristic yellow resonance line of sodium, commonly called the D line and consisting of a doublet whose components are emitted at 5896 and 5890 A. Since the peak of the sodium D line falls close to the peak of the sensitivity curve of the human eye at about 5600 A.,' the result is a relatively efiicient lamp and efiiciencies in excess of 100 lumens per watt may be achieved. Unfortunately, however, the low pressure sodium lamp is a substantially monochromatic light source. The human complexion under its light has a rather ghastly appearance and the lamp is hardly suitable for general illumination.

The general object of the invention is to provide a new and improved sodium vapor lamp operating at high efiiciency and providing a color rendition making it suitable for general illumination applications.

More specific objects of the invention are to provide improvements in the efiiciency and color rendition of sodium vapor lamps operating at relatively high pressures and current densities.

In accordance with the invention, 1 have discovered that a high efliciency light source with good color rendition suitable for general illumination results from a relatively high current density discharge through sodium vapor when its vapor pressure is increased in excess of 30 millimeters of mercury pressure. Current densities in the range of 4 to 70 amperes per square centimeter of discharge cross section are desirable, and the range of 7 to 45 amperes per square centimeter is preferred. By contrast with the yellow monochromatic light output of the low pressure sodium lamp, the light from this relatively high pressure sodium lamp has a white or nearly white color. The lamps may use either saturated or unsaturated sodium vapor. In the saturated vapor lamp wherein an excess amount of sodium is placed in the lamp, a temperature in excess of 600 C. is required for the coldest spot within the lamp structure in order to obtain the desired vapor pressure. In the unsaturated vapor lamp, the quantity of sodium placed in the lamp is measured so that after complete vaporization during the warming-up period, the desired sodium vapor pressure results at operating temperature.

A rather surprising feature of the high pressure sodium vapor lamps according to the invention is that efficiency in the production of visible light does not continue to rise with increasing vapor pressure, as one might be inclined to expect on the basis of experience with high pressure mercury vapor lamps. Rather a peak or maximum efficiency is realized at a sodium vapor pressure in the range. of 30 to 500 millimeters of mercury pressure, the specific occurrence of the peak depending upon factors including current density, choice of fill gas, and fill gas pressure. In the case of the saturated vapor lamp, the pressure range of 30 to 500 millimeters of mercury corresponds to a temperature range of 600 to 820 C. for the coldest spot in the lamp structure which determines the sodium vapor pressure.

According to another feature of the invention, I have found that maximum efficiency is greatly influenced by the nature and pressure of the inert fill gas. The heaviest inert gas in respect of atomic weight is xenon and it is definitely preferable from the point of view of peak efiiciency in lumens per watt.

I have determined that maximum efficiency occurs at a sodium vapor pressure of approximately millimeters of mercury in conjunction with a xenon fill pressure of about 210 millimeters of mercury, the peak efficiency then being about lumens per watt in the case of a 6 millimeter internal diameter lamp operating at a current of about 12 amperes. However, the relatively low sodium vapor pressure at optimum efiiciency is reflected in a more yellowish emission. Improved color rendition may be achieved by increasing the sodium vapor pressure, but at a lower efiiciency. In practice, the drop in efilciency which occurs simultaneously with the improvement in color rendition as the vapor pressure is increased sets about 1000 millimeters of mercury pressure as the upper limit in lamps for general use. At the higher sodium vapor pressures, there is little advantage from the point of view of efliciency in a high Xenon fill pressure. Since a high xenon fill pressure entails higher starting voltage and imposed a heavier burden on the lamp seals, a relatively low xenon fill pressure, for instance approximately 20 millimeters of xenon, is preferred under these conditions.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following description of a preferred embodiment taken in conjunction with the accompanying drawings. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawings:

FIG. 1 is a sectional view of a sodium vapor lamp embodying the invention.

FIGS. 2a to 20. are graphs illustrating the spectral characteristics of the lamp at various sodium vapor pressures.

FIG. 3 is a graph illustrating the peaking of luminous efficiency with sodium vapor pressure at different current loadings in a lamp embodying the invention.

FIG. 4. illustrates graphically the peaking of luminous efficiency with xenon fill pressure and the corresponding sodium vapor pressure and lamp current.

FIG. illustrates graphically relative red, green and blue emission over a range of sodium vapor pressure.

FIG. 6 illustrates graphically relative luminous efficiency over the useful range of sodium vapor pressure for a relatively low xenon fill pressure of 20 millimeters of mercury.

FIG. 7 illustrates graphically relative luminous efficiency vs. sodium vapor pressure for an intermediate xenon fill pressure of 150 millimeters of mercury.

FIG. 8 illustrates graphically relative luminous efficiency vs. sodium vapor pressure for a relatively high xenon fill pressure of 200 millimeters of mercury.

The envelope of the lamp must be of a material which will withstand the attack of the sodium vapor at the relatively high operating pressures and temperatures. Ordinary glasses and quartz are not suitable and even the sodium resistant glazes which are used to line the inside of the envelopes of low pressure sodium discharge lamps cannot withstand the attack of the sodium vapor at high temperatures. Therefore it is necessary to resort to envelopes of other materials such as ceramics. A suitable material is disclosed in my copending application 836,200, filed August 26, 1959, entitled Metal Vapor Lamps, and assigned to the same assignee as the present invention, now Patent No. 2,971,110 issued Feb. 7, 1961.

That material consists of sintered transparent polycrystalline alumina consisting essentially of aluminum oxide and having a melting point not less than 1925 'C. The material has a very high alumina content, for instance in excess of 99.5% A1 0 and though translucent, rather than clear like glass, has exceedingly good light transmittance, in excess of 95%. Furthermore, as disclosed in said copending application, the sintered high density polycrystalline alumina can withstand the vapors of the alkali metals without blackening for long periods of time even at temperatures as high as 1600 C.

Referring to FIG. 1, a lamp 1 embodying the invention comprises an envelope 2 of ceramic tubing of sintered transparent polycrsytalline alumina. The inside diameter of the tubing is uniform throughout its length but the outside diameter is expanded at the ends to facilitate brazinz closure members thereto. Metal caps or closure members 3, 4 consisting of a nickel-chromium-iron alloy having a high temperature melting point and a coefficient of expansion close to that of the alumina are brazed to the ends of the tubing using thin titanium washers to metallize the ends of the tube in order to bond the discs thereto. Alumina back-up rings 5, 6, in effect short sections of tubing of the same diameter and wall thickness as the ends of the tubing 1, are brazed to the outer faces of the caps 3,4, again through the use of titanium Washers to metallize the surface of the ceramic rings. The purpose of the back-up rings is to balance the strains set up between the caps 3, 4 and the alumina parts to which they are brazed throughout the temperature range to which the ends of the lamp are subjected in operation. The brazing of the end caps may be done in a vacuum or in a reducing or inert atmosphere at a suitable temperature, for instance between 900 and 1000" C.

A metal tube 7, suitably of stainless steel or of iron nickel chromium alloy, passes through a central perforation in cap 3 at one end of the lamp and is brazed to it to make a hermetic seal. Tube 7 supports on its inner end a cathode 8 consisting of a double wound tungsten wire coil with the interstices filled with activating material in the form of alkaline earth oxides including barium oxide. The tungsten coils forming the cathode are wound over a tungsten shank 9 which is welded in the end of the metal tube 7. Electrode 10 at the other end of the tube is supported from a short length of metal tube 11 welded to the inside face of end cap 4.

The tube 7 is used to evacuate the lamp and to introduce the ionizable medium therein comprising the sodium and also an inert starting gas to facilitate starting. The sodium may be introduced into the envelope by placing a quantity thereof in a thin-walled glass capsule and placing the capsule in an extension (not shown in the drawing) of the tube 7. The envelope is then evacuated through the tube 7, the side aperture 12 therein permitting ready passage of gas; the inert starting gas is introduced in the same way, and the end of the tube extension is then closed off by pinch-welding. The glass capsule is broken by squeezing the tube extension which is heated to drive the sodium into the lamp envelope. The tube 7 may then be pinch-welded a second time closer to cap 3, as indicated at 13 in the drawing, and the extended portion containing the glass fragments broken off and discarded, leaving the lamp as illustrated.

Alternatively, a lamp envelope similar in physical configuration and electrical characteristics may be used but having generally thimble-shaped niobium end caps inserted axially into the ends of the ceramic tube and sealed by a thin layer of glassy material of a near eutectic mixture comprising mainly aluminum oxide and calcium oxide. This construction is described and claimed in copending application Serial No. 247,583, filed December 27, 1962, of William C. Louden and Richard S. Pinter, entitled Ceramic Lamp Construction, and assigned to the same assignee as the present invention.

The inert gases which may be used for starting purposes in electric discharge lamps are, in increasing order of atomic weight, helium, neon, argon, krypton and xenon. The gas which has been used in the low pressure yellow sodium lamp is neon. I have determined that in the relatively high pressure near white color sodium lamp of the invention, the heavier inert gases argon, krypton and xenon or mixtures thereof, provide an appreciable advantage in efficiency, 10% or more, over the lighter gas neon. The heaviest inert gas xenon is, by a wide margin, the most desirable from the points of view of efiiciency. Since xenon is expensive, for reasons of economy krypton or argon may be used alone or admixed with xenon, but generally at some sacrifice in efficiency.

The specific xenon pressure for maximum efiiciency in a given lamp design will depend on factors such as are tube diameter, current density, and in particular on sodium vapor pressure. In FIG. 4, curve 41 shows how the peak luminous efiiciency varies with xenon fill pressure over the range of interest from 5 to 300 millimeters of mercury; curve 42 shows the corresponding sodium vapor pressure and curve 43 shows the corresponding lamp current. The data were obtained from lamps of the same geometry as illustrated generally in FIG. 1 but with different xenon cold filling pressures. The lamps had an internal diameter of about 6 millimeters and an inter-electrode gap of about 60 millimeters. A series of tests was performed for each lamp with operating currents varying between 6 and 18 amperes and sodium vapor pressures varying from 13 to 420 millimeters of mercury. From these results, the maximum elficiency for each lamp with a definite xenon filling pressure was selected and plotted as a function of xenon filling pressure to give curve 41; the corresponding sodium vapor pressures and currents were plotted to give curves 42 and 43, respectively.

It will be observed that maximum efficiency increases with increasing xenon fill pressure from a minimum of about lumens per watt at about 10 millimeters of xenon to a maximum of about lumens per watt at approximately 210 millimeters of xenon. At the same time the corresponding sodium vapor pressure drops from about millimeters to about 75 millimeters. The lower sodium 'vapor pressure at peak luminous efficiency is reflected in a more yellowish emission of the discharge. At high sodium vapor pressures, the lamp has good color rendition due to pronounced self-adsorption of the yellow D lines and strong broadening into the red and green course does not contribute to visible light.

regions of the spectrum. In lowering the vapor pressure, these effects are reduced in magnitude and the emission becomes more yellowish.

I believe that the reason for the large increase in efficiency with increasing xenon fill gas pressure is due to the low coefficient of heat condition of xenon. The main losses of energy in the arc column are radiation and heat condition to the wall. During lamp operation, there exists essentially a two-component system of sodium vapor and fill gas. By choosing for the fill gas xenon which has low heat conductivity, heat conduction is minimized and luminous efiiciency is increased. The effective heat conductivity in a two-component system is determined of course by the composition of the mixture. Increasing the xenon content entails a decrease in effective heat conductivity which in turn is manifested as an increase in luminous efficiency. However this effect reaches a maximum, observed at about 210 millimeters of mercury, due to the counter-balancing effects of increasing elastic losses.

In tests of lamps embodying the invention and constructed according to FIG. 1, a great improvement in color rendition was observed with increasing sodium vapor pressure. Rather surprisingly and contrary to expectations, no strong continuum appears in the visible. With increasing vapor pressure one would expect the excitation of higher transitions to increase, thereby enhancing the emission of lines other than the resonance line (sodium D line). This effect has been observed but the strongest higher transistions appear in the infrared at about 8194 A. and 11,400 A.; in the visible, line emission is only slightly enhanced at 5688 A. and 6160 A. and at some other wave lengths where the magnitude is not significant. Thus the explanation for the improvement in color rendition with increasing sodium vapor pressure must reside elsewhere than in increased line radiation and I have found that in lines in self-reversal and broadening of the yellow resonance line.

The foregoing is illustrated in FIG. 2 showing the spectral distributions in the visible range from 4000 to 7000 A. of a lamp as illustrated in FIG. 1 for various sodium vapor pressures. In FIG. 2a, the sodium vapor pressure is 3- 10- millimeters of mercury and the pronounced peak at 5890 A. corresponding to the sodium D line results in a substantially monochromatic yellow light. In FIG. 2b, the sodium vapor pressure is 4'millimeters of mercury; enhancement of the lines at 6160 and 5690 A. has occurred, but more important, broadening and selfreversal of the sodium D line has begun. In FIG. 2c, the sodium vapor pressure is 20 millimeters of mercury and the previously noted broadening and self-reversal of the sodium D line has increased. In FIG. 2d, the sodium vapor pressure is 200 millimeters of mercury; the sodium D line is completely absorbed and the wings have broadened throughout a substantial portion of the visible spectrum.

In FIG. 2d, the spectral distribution shows a good color rendition and the color of the light from the lamp appears to be nearly white. If the sodium vapor pressure is increased substantially in excess of the indicated value (200 millimeters), further broadening of the self-reversal gap around 5900 A. and further broadening of the. line wings, preferentially towards the infrared, occurs. However the self-reversal gap occurs close to the peak of the sensitivity curve of the human eye and the infrared radiation of Thus even though the total radiant output many continue to increase with increasing sodium vapor pressure, the output in the visible range decreases. This is believed to explain the occurrence ofa peak in luminous efiiciency in the pressure range from 30 to 500 millimeters of mercury pressure.

The peaking in luminous efficiency in the sodium vapor pressure range of 30 to 500 millimeters of mercury is illustrated in FIG. 3. The measurements were taken on a lamp as illustrated in FIG. 1 wherein the inside diameter of the envelope was about 6 millimeters and the inter electrode gap or distance between the ends of the electrodes was about 60 millimeters. The lamp was filled with an excess of sodium and argon at 20 millimeters of mercury; the sodium vapor pressure was controlled by regulating the temperature of the coldest spot in the lamp structure. The lamp efliciency is plotted in arbitrary units as a function of cold spot temperature and the corresponding sodium vapor pressure is also indicated on the abscissa scale. Curve 20 shows the results for an operating current of 5 amperes corresponding to a current density of timeter length in the interelectrode gap. Curve 21 shows the results for an operating current of 10 amperes corresponding to a current density of approximately 36 amperes per square centimeter. The input loading increases from 400 to 900 Watts through the stated range; this corresponds to a loading of 67 to watts per centimeter length of interelectrode gap.

The occurrence of the peak in efficiency or luminous output depends upon the sodium vapor density, fill gas pressure, and also upon other factors such as the current density, that is the actual operating current in relation to the cross section of the envelope. It is also affected by the presence of auxiliary discharge supporting gases or vapors in the ionizable medium, such as for instance mercury and thallium which will be discussed in greater detail hereinafter. In general however the peak in efiiciency will occur in the sodium vapor pressure range from 30 to 500 millimeters of mercury, corresponding to a control temperature range of 600 to 820 C. for a saturated vapor lamp. For the two examples whose performance curves are shown in FIG. 3, the peak in efficiency occurs at about 200 millimeters of mercury corresponding to a control temperature of about 730 C.

The sodium vapor pressure range of interest. in the design of lamps embodying the invention extends from about 30 millimeters of mercury to 1000 millimeters of mercury. This range, which may be termed the region of high efficiency in the visible spectrum, may conveniently be divided into three regions identified and characterized as follows.

Range of sodium vapor pressure: Characteristics 30 to 200 millimeters of mercury Maximum efiiciency.

200 to 300 millimeters of mercury Good color rendition at decreased efliciency.

300 to 1000 millimeters of mercury Optimum color rendition.

FIG. 5 presents in graphical fashion data on the emission of the lamp in different spectral regions as a function of sodium vapor pressure. The data were obtained by directing the light from the lamp on a photocell corrected to the eye sensitivity curve. Different spectral re gions in the red, green and blue were selected by inserting between the lamp and the photocell standard glass filters identified respectively CS 2-61, CS 4-64, and CS 5-61 (Corning). The curves are normalized at a sodium vapor pressure of 4 millimeters of mercury, that is each curve, 51 for the red, 52 for the green, and 53 for the blue is assigned a value of unity at this pressure. The gain in emission in the different wave length or color bands is plotted as a function of sodium vapor pressure. At. a gain of about l0.2 in the red band (correspond ing to a vapor pressure of about 230 millimeters of mercury), the red emission of the lamp is equivalent to that of sunlight. With increasing sodium vapor pressure, the gain in red emission begins to level olf at about 700 millimeters, and it reaches a saturation value of about 20 in the range from 800 to 1000 millimeters. In the region above 300 millimeters the lamp exhibits exaggerated red rendition which is somewhat balanced by the increased green and blue emission, resulting in a white over-all lurninous radiation.

In FIG. 6, curves 61, 62 and 63 show, respectively, the relationship between relative luminous efficiency and sodium vapor pressure for operating currents of 6 amperes, 8 amperes and 10 to 12 amperes, respectively, with xenon as the fill gas at a pressure of 20 millimeters of mercury. Similarly, in FIG. 7, curves 71, 72 and 73 show the relationship of relative luminous efficiency to sodium vapor pressure for the indicated currents with xenon at a fill pressure of 150 millimeters of mercury. In FIG. 8, curves 81, 82 and 83 show the corresponding relationship with xenon at a fill pressure of 200 millimeters of mercury. Comparing the three sets of curves, it will be observed that the highest efiiciencies are indicated in FIG. 8 where a high xenon fill pressure is used (200 mm.), but a relatively low sodium vapor pressure (75 to 100 mm.) is simultaneously required. The relative efficiency is lower in FIG. 7 and still lower in FIG. 6, but the corresponding sodium vapor pressure is simultaneously increasing. Thus in FIG. 6 for a xenon fill pressure of 20 millimeters, at a current of 6 amperes the maximum in luminous efiiciency occurs at a sodium vapor pressure of 200 millimeters of mercury. Since color rendition improves with increasing sodium vapor pressure, there is no advantage in such case in going to the high xenon fill pressure and a fill pressure of 20 millimeters of mercury is sufficient or even preferable. The higher xenon fill pressures entail higher starting voltages, an undesirable effect, and also impose a heavier burden on the lamp structure and particularly on the seals. Also it will be observed that with the higher xenon fill pressure (FIG. 8), the relative luminous efiiciency curves fall off more steeply with higher sodium vapor pressure. When all of the foregoing factors are taken into consideration, I have found it is generally preferable to use a xenon fill pressure of about 20 millimeters unless maximum luminous efiiciency is the prime consideration, in which case a higher xenon fill pressure may be resorted to.

According to a further feature of the invention, I have found that an increase in efiiciency may be achieved by the addition of mercury to the sodium vapor discharge medium. I have made lamps with measured quantities of mercury added to an excess amount of sodium which showed efiiciencies greater than those of similar lamps with sodium alone. The amount of mercury added should be determined on the basis of complete vaporization during operation and thereafter exertion of a partial pressure of mercury vapor in the range of 1 to atmospheres. It will be appreciated that after complete vaporization, the partial pressure continues to increases with temperature, but more slowly in accordance with the gas law, that is following the relationship that the product of pressure by volume varies as the absolute temperature, as commonly denoted by the expression PV=RT. It may be noted that even though the partial pressure of mercury is several times greater than the partial pressure of sodium, the lamp nevertheless remains basically a sodium vapor discharge lamp. This is due to the fact that the ionization potential of sodium at 5.1 volts is lower than that of mercury at 10.4 volts, with the result that the current carriers are furnished by the sodium. Also the first excitation potential of sodium at 2.1 volts is much lower than the first excitation potential of mercury at 4.9 volts; in consequence, excitation of sodium atoms occurs at a much greater rate than excitation of mercury atoms. From the foregoing it follows that in lamps according to the invention, sodium atoms are both the principal discharge supporting element and principal source of radiant emission.

As an example of a sodium lamp with added mercury, one particular lamp constructed having an internal diameter of 9 millimeters and an interelectrode gap of 56 millimeters was provided with a filling consisting of an excess amount of sodium, 60 milligrams of mercury, and argon at 20 millimeters pressure for a starting gas. This lamp was operated so that the sodium developed a partial pressure of 200 millimeters of mercury, the corresponding partial pressure of mercury being about 5 atmospheres; the measured efficiency was 103 lumens per watt with near-white emission. A similar lamp not having the addition of mercury and operated under similar conditions, that is at the same pressure of sodium, had a measured efiiciency of 91 lumens per watt. Thus the improvement in efliciency in the first case due to the addition of mercury was about 12%.

According to another feature of the invention, the sodium vapor discharge lamp may also be improved in efiiciency and color rendition by the addition of thallium metal or preferably by the addition of both mercury and thallium metal. The thallium green line which has an excitation energy of about 3.3 electron volts, is excited in the sodium arc and contributes favorably to the luminous efficiency. In order to obtain best yield from the thallium addition, a partial pressure of thallium in the range from 7 10- to 6x10 millimeters of mercury pressure appears desirable. This pressure range corresponds to a control temperature range of about 800 to 1400 C. Except at the lower limit of this range, the resulting pressure of sodium vapor in a saturated vapor type lamp would be excessive. Accordingly an unsaturated vapor type of lamp is used wherein a limited amount of sodium is placed in the lamp envelope and completely vaporized, the sodium vapor then increasing with further increase in temperature according to the gas law to give the desired partial vapor pressure at the operating temperature. If mercury is added to the sodium, in addition to the thallium, it must also be added in a limited amount, which, after complete vaporization, will result in the desired partial pressure of mercury vapor at the operating temperature within the stated range of 800 to 1400 C.

The foregoing examples of the invention are intended as illustrative and not in order to limit the invention thereto except inasmuch as specific limitations may be appear in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge lamp comprising an envelope of light-transmitting material resistant to the attack of sodium vapor at elevated temperatures, a pair of electrodes sealed therein, and an ionizable medium within said envelope comprising sodium as the principal discharge supporting element and source of radiant emission and developing a partial pressure of sodium vapor in the range of 30 to 1000 millimeters of mercury in operation of said lamp corresponding to the region of high efiiciency in the visible spectrum resulting from self- 1reversal and broadening of the yellow sodium resonance 2. An electric discharge lamp comprising an envelope of light-transmitting material resistant to the attack of sodium vapor at elevated temperatures, a pair of electrodes sealed therein, and an ionizable medium within said envelope comprising sodium as the principal discharge supporting element and source of radiant emission and developing a partial pressure of sodium vapor exceeding 30 millimeters of mercury in operation of said lamp at current densities in the range of 4 to 70 amperes per square centimeter of discharge cross section corresponding to the region of high efiiciency in the visible spectrum 9 resulting from self-reversal and broadening of the yellow sodium resonance line.

3. An electric discharge lamp comprising an envelope of light-transmitting ceramic material resistant to the attack of sodium vapor at elevated temperatures, a pair of electrodes sealed therein, and an ionizable medium Within said envelope comprising sodium vapor as the principal discharge supporting element and source of radiant emission at a partial pressure in the range of 30 to 1000 millimeters of mercury in operation of said lamp, said sodium being the element with lowest excitation and ionization potentials in said medium, said range corresponding to the region of high efiiciency in the visible spectrum resulting from self-reversal and broadening of the yellow sodium resonance line.

4. An electric discharge lamp comprising an envelope of light-transmitting material resistant to the attack of sodium vapor at elevated temperatures, a pair of electrodes sealed therein, and an ionizable medium within said envelope comprising sodium as the principal discharge supporting element and source of radiant emission and developing a partial pressure of sodium vapor in the range of 30 to 1000 millimeters of mercury in operation of said lamp at current densities in the range of 4 to 70 amperes per square centimeter of discharge cross section, said sodiumbeing the element with lowest excitation andionization potentials in said medium.

5. An electric discharge lamp comprising an envelope of light-transmitting material resistant to the attack of sodium vapor at elevated temperatures, a pair of electrodes sealed therein, and an ionizable medium within said envelope comprising an inert starting gas from the group consisting of argon, krypton, xenon and mixtures thereof at a filling pressure in the range of 5 to 300 millimeters of mercury, and sodium as the principal source of radiant emission and developing a partial pressure of sodium vapor in the range of 30 to 1000 millimeters of mercury in operation of said lamp at current densities in the range of 4 to 70 amperes per square centimeter of discharge cross section.

6. A sodium vapor discharge lamp having a radiant emission Widely distributed throughout the visible spectrum comprising a tubular elongated envelope of material resistant to the attack of sodium vapor at high temperatures, a pair of electrodes sealed into opposite ends, an

ionizable medium in said envelope comprising a quantity emission widely distributed throughout the visible spectrum comprising a tubular elongated envelope of material resistant to the attack of sodium vapor at high temperatures, a pairof electrodes sealed into opposite ends, an ionizable medium in said envelope comprising sodium whereof the vapor serves as the principal discharge supporting constituent and source of radiant emission, said sodium being limited to a quantity totally vaporized and the vapor thereof exerting a partial pressure in the range of 30 to 1000 millimeters of mercury in operation of said lamp at current densities in the range of 4 to 70 ampere-s per square centimeter of discharge cross section.

8. An electric discharge lamp producing a spectral emission widely distributed throughout the visible range comprising a tubular elongated envelope of material resistant to the attack of sodium vapor at high temperatures, a pair of electrodes sealed into opposite ends, an ionizable medium within said envelope comprising an inert starting mercury and being the principal discharge supporting element and source of radiant emission, and said mercury exerting a partial vapor pressure in the range of 1 to 15 atmospheres in operation of said lamp at current densities in the range of 4 to 70 amperes per square centimeter of discharge cross section.

9. An electric discharge lamp producing a spectral emission widely distributed throughout the visible range comprising a tubular elongated envelope of material resistant to the attack of sodium vapor at high temperatures, a pair of electrodes sealed into opposite ends, an ionizable medium Within said envelope comprising an inert starting gas, in excess of that vaporized in operation sodium metal, and a limited quantity of mercury, the sodium exerting a partial pressure in the range of 30 to 500 millimeters of mercury during normal operation of said lamp wherein the coldest spot in said lamp envelope is maintained at a temperature in the range of 600 to 820 C., said sodium being the principal discharge supporting element and source of radiant emission, and said quantity of mercury being limited to exert, when totally vaporized under normal operating conditions of said lamp, a partial vapor pressure in the range of l to 15 atmospheres.

10. An electric discharge lamp producing a spectral emission widely distributed throughout the visible range comprising a tubular elongated envelope of material resistant to the attack of sodium vapor at high temperatures, a pair of electrodes sealed into opposite ends, an ionizable medium within said envelope comprising an inert starting gas, sodium, mercury, and thallium, said sodium exerting a partial pressure in the range of 30 to 1000 millimeters of mercury and being the principal discharge supporting element and source of radiant emission, said mercury exerting a partial vapor pressure in the range of l to 15 atmospheres, and said thallium exerting a partial pressure in the range of 7 l0 to 6 l0 millimeters of mercury, in operation of said lamp at current densities in the range of 4 to 70 amperes per square centimeter of discharge cross section.

- 11. An electric discharge lamp comprising a tubular envelope of high density polycrystalline alumina, a pair of electrodes sealed into opposite ends, and an ionizable medium within said envelope comprising xenon at a fill pressure in the range of 5 to 300 millimeters of mercury, and sodium as the principal discharge supporting element and source of radiant emission developing a partial vapor pressure in the range of 30 to 1000 millimeters of mercury in operation of said lamp at current densities in the range of 4 to 70 amperes per square centimeter of discharge cross section.

12. A sodium vapor discharge lamp for maximum luminous efiiciency comprising a tubular envelope of high density polycrystalline alumina, a pair of electrodes sealed into opposite ends, and an ionizable medium within said envelope comprising xenon at a fill pressure of about 210 millimeters of mercury, and sodium as the principal discharge supporting element and source of radiant emission developing a partial vapor pressure of approximately 75 millimeters of mercury in operation of said lamp.

13. A sodium vapor discharge lamp having high efficiency comprising an envelope of high density polycrystalline alumina, a pair of electrodes sealed into opposite ends, and an ionizable medium within said envelope comprising xenon at a fill pressure of approximately 20 millimeters of mercury, and sodium as the principal discharge supporting element and source of radiant emission developing a partial vapor pressure in the range of 30 to 200 millimeters of mercury in operation of said lamp.

14. A sodium vapor discharge lamp for good color rendition comprising an envelope of high density polycrystalline alumina, a pair of electrodes sealed into opposite ends, and an ionizable medium within said envelope comprising xenon at a fill pressure of approximately 20 millimeters of mercury, and sodium as the principal ll 1 1 2 source of radiant emission developing a partial vapor developing a partial Vapor pressure in the range of 300 pressure in the range of 200 to 300 millimeters of merto 1000 millimeters of mercury in operation of said lamp.

cury in operation of said lamp.

15. A sodium vapor discharge lamp for optimum color References Cited by the Examiner rendition comprising an envelope of high density poly- 5 UNITED STATES PATENTS crystalline alumina, a pair of electrodes sealed into oppo- 033 9 1932 Spaeth 3 site ends, and an ionizable medium within said envelope 2 1 1 324 1939 K fit 313 221 comprising xenon at a fill pressure of approximately 20 2 990 490 6/1961 HeineGeldem 313 134 X millimeters of mercury, and sodium as the principal discharge supporting element and source of radiant emission 10 GEORGE WESTBY, Primary Examine"- Disclaimer .-Km"t Schmidt, Cleveland Height SODIUM VAPOR LAMP. Patent clalmer filed June 2-1, 1966, by the inventor; E Zectm'c Company, consenting. Hereby enters this disclaimer to claim 9 of said patent.

[Ofim'al Gazette August 9, 1.966.]

s, Ohio. HIGH PRESSURE dated Apr. 26, 1966. Disthe assignee, Geneml 

1. AN ELECTRIC DISCHARGE LAMP COMPRISING AN ENVELOPE OF LIGHT-TRANSMITTING MATERIAL RESISTANT TO THE ATTACK OF SODIUM VAPOR AT ELEVATED TEMPERATURES, A PAIR OF ELECTRODES SEALED THEREIN, AND AN IONIZABLE MEDIUM WITHIN SAID ENVELOPE COMPRISING SODIUM AS THE PRINCIPAL DISCHARGE SUPPORTING ELEMENT AND SOURCE OF RADIANT EMISSION AND DEVELOPING A PARTIAL PRESSURE OF SODIUM VAPOR IN THE RANGE OF 30 TO 1000 MILLIMETERS OF MERCURY IN OPERATION OF SAID LAMP CORRESPONDING TO THE REGION OF HIGH EFFICIENCY IN THE VISIBLE SPECTRUM RESULTING FROM SELFREVERSAL AND BROADENING OF THE YELLOW SODIUM RESONANCE LINE. 